Inertial navigation is the process of navigating a body (such as an aircraft) using an Inertial Reference System (“IRS”), which is coupled to the body. To do this, the IRS may calculate a navigation solution of the body using its self-contained or “un-aided” sensors, such as accelerometers, gyroscopes and barometric sensors. As described in more detail below, the navigation solution may, depending on the operational mode of the IRS, include all or portions of an attitude, position and velocity of the body.
Before navigation can commence, the IRS typically undergoes a self-alignment process that establishes a relationship between coordinates of the body and a local geographic reference. This self-alignment process generally includes the functions of (i) leveling the IRS and (ii) determining an initial azimuth of the IRS, which is also known as gyrocompassing. To perform the leveling function, the IRS may align, using its unaided sensors (e.g., gyroscopes and accelerometers), a vertical axis of its local-level-coordinate frame with a sensed acceleration vector. To determine the initial azimuth, the IRS may measure a horizontal earth rate.
To carry out the self-alignment process, the IRS (and the body to which it is attached) has to remain substantially stationary for a period of time. Such process is commonly and aptly named the “stationary-alignment mode.” The stationary-alignment mode is the typical alignment mode for the IRS, and is performed each time power is applied to the IRS so long as the IRS remains stationary for the duration of the stationary alignment. A typical duration for the stationary-alignment mode may be, for example, between 3 and 17 minutes. The duration, however, may be less than or greater than this exemplary range, and may be a function of latitude.
After the stationary-alignment mode is completed, the IRS device may transition to a navigation mode. In the navigation mode, the IRS supplies full navigational parameters for the body to which it is attached. That is, the IRS supplies an attitude, position and velocity of the body.
In addition to the stationary-alignment and navigation modes, the IRS may be operated in a reversionary-attitude mode to allow quick recovery of attitudes, body rates, and body accelerations in the event of an interruption of power or excessive movement during the stationary-alignment mode. The reversionary-attitude mode, however, is not used to align the IRS, but rather allows for continued navigation with less than full navigational parameters. To allow such continued navigation, the reversionary-attitude mode may implement a non-accelerating assumption, via, for example, a second order control loop, to maintain a leveled platform to allow the IRS to derive pitch and roll attitudes.
In reversionary-attitude mode, however, the IRS does not provide a valid self-computed heading output. Instead, the heading output is set to a null condition, and remains in this condition until it receives an initial set heading input. An initial set heading (magnetic, true or otherwise) may be input into the IRS as a reference for the derived platform heading. The set heading may be input periodically to maintain an accurate heading output while the IRS is in the reversionary-attitude mode.
Typically, the IRS is powered by two power sources. When both of these sources are interrupted for a predetermined period, e.g., greater than 20 milliseconds, the IRS is generally reset. On reset, the IRS (through user interaction) may enter (i) the stationary-alignment mode before transitioning to the navigation mode (assuming navigation mode, in fact, can be attained) or (ii) the reversionary-attitude mode and continue to operate without the ability to transition to the navigation mode.
If, for example, the IRS undergoes a power interrupt and is reset while the body is stationary, then the user may cause (e.g., by flipping a switch) the IRS to enter the stationary-alignment mode. Once the stationary-alignment mode is completed, the IRS may transition to the navigation mode via user interaction. The reversionary-attitude mode, however, is typically used when the interruption of power occurs while the body is in motion. However, the reversionary-attitude mode may be used if the body undergoes excessive movement before the stationary-alignment mode is completed.